In Neurospora, the arginine pathway is a discrete, well-studied eucaryotic metabolic system in which both compartmental and kinetic features prevail. Our intent is to probe this system as a model of the interplay of these features in metabolic flow. The proposal is divided into two parts. In the first part, the mitochondrial enzyme, carbamyl phosphate synthetase A, is discussed. This enzyme has two polypeptides and has evolved such that it is not feedback inhibited by arginine. Instead, it is uniquely (and highly) regulated by repression, according to our present evidence. We wish to confirm its indifference to arginine, to study the regulatory circuit governing its synthesis, and to study the aggregation of polypeptides and their insertion into mitochondria. The second part of the proposal concerns ornithine distribution in mitochondria, vacuoles and cytosol, and its flow into the arginine, polyamine, and proline pathways. Previous isotope tracer work has shown how the three compartments contribute to the three pathways in wild-type Neurospora. This work proposed seeks to study genetic variants in which ornithine allocations to different pathways are altered, and the kinetic or compartmental basis of the differences.